Transformers make up a large portion of power delivery systems throughout the world and are the backbone of electric power conversion systems. The positive attributes of conventional distribution transformers have been well documented for years and include low cost, high reliability, and high efficiency. Were it not for these highly reliable devices, the distance separating generators from consumers would have been significantly greater. Indeed, to distribute power over such distances would require many households and industries to operate their own substations, resulting in electricity being a much less practical form of energy.
Like other devices in modern electrical distribution systems, the conventional transformer has some drawbacks. The drawbacks include voltage drop under load, inability to mitigate “flicker,” sensitivity to harmonics, environmental impacts when mineral oil leaks occur, limited performance under DC-offset load unbalances, inability to convert single-phase service to three-phase for powering certain types of equipment and no energy-storage capacity. One consequence of not having energy storage capacity is that the output can be easily interrupted because of a disturbance at the input. Also, when the output load current generates harmonics and reactive power, the conventional transformer reflects them back to the input side.
Power-line disturbances, such as voltage sags and momentary interruptions, cost electric utility customers billions of dollars every year. With today's increased complexity of process automation, even if only a small segment of a process is vulnerable, power disruptions may interrupt the entire automated process. In response to this perceived market need, a broad range of distribution-class, power-conditioning devices, commonly known as “custom power” devices have been introduced in the market. These custom power devices include reactive power and harmonic compensation devices, such as the Adaptive Var Compensator (AVC) and voltage sag and momentary interruption protection devices, such as a Dynamic Voltage Restorer (DVR). Unfortunately, the customer response to these new lines of products has not been receptive, mainly because of their cost.